(1) Field of the Invention
The present invention relates to nozzles for coupling to a ladle in a metal casting installation (in particular to a steel casting installation), referred to as ladle shrouds. In particular, it relates to ladle shrouds which can be loaded to and unloaded from the bottom base of a ladle, slipped into casting position and which can maintain their casting position without any external means such as a manipulator or a robot. The present invention also concerns a kit of part for a coupling assembly allowing such reversible coupling, a metal casting installation comprising such nozzle, and a process of coupling a ladle shroud to the bottom base of a ladle.
(2) Description of the Related Art
In metal forming processes, molten metal is transferred from one metallurgical vessel to another, to a mould or to a tool. For example, as shown in FIGS. 1A and 1B a ladle (11) is filled with molten metal out of a furnace and transferred over a tundish (10) to cast the molten metal through a ladle shroud (111) into said tundish. The molten metal can then be cast through a pouring nozzle (101) from the tundish to a mould for forming slabs, billets, beams or ingots. Flow of molten metal out of a metallurgic vessel is driven by gravity through a nozzle system (101, 111) located at the bottom of said vessel.
In particular, the inner surface of the bottom floor of a ladle (11) is provided with an inner nozzle (113) comprising an inner bore. The outlet end (113b) of said inner nozzle is coupled to a gate (114u, 114d), generally a sliding gate or a rotary gate, controlling the flow of molten metal out of the ladle. In order to protect the molten metal from oxidation as it flows from the ladle to a tundish (10), a ladle shroud (111) is brought in fluid communication (via its upper end) with the outlet end of the inner nozzle while its lower end is immersed into the tundish, generally below the level of molten metal; to form a continuous molten metal flow path shielded from any contact with oxygen between the inlet end (113a) of the inner nozzle (113) within the ladle down to the outlet of the ladle shroud immersed in the liquid metal contained in the tundish. A ladle shroud is simply a nozzle comprising a long tubular portion crowned by an upstream coupling portion with a central bore. In many cases, the ladle shroud is inserted about and sealed to a short collector nozzle (110) coupled to, and jutting out of the outer surface of the ladle bottom floor, and which is separated from the inner nozzle (113) by a gate (114u, 114d).
In practice, a ladle is brought to its casting position over a tundish or a mould from a furnace, a converter or another ladle where it was filled with a batch of molten metal, with the gate (114u, 114d) in a closed configuration. During its trips from the furnace, converter or other ladle to the casting position over a tundish and back, the ladle is not coupled to any ladle shroud (111) because the latter is long and it would be dangerous to move a ladle back and forth across a workshop with a long ladle shroud jutting out of its lower base. Once the ladle is at its casting position above a tundish (10), a manipulator or a robot (20) brings a ladle shroud into casting configuration. As shown in FIG. 1B, in traditional casting installations, the outlet end of the collector nozzle (110) is snugly nested in the bore inlet of the ladle shroud to form a sealing joint. The manipulator or robot (20) must maintain the ladle shroud (111) in its casting configuration during the whole casting of the molten metal batch contained in the ladle (11). When the ladle is empty, the gate is closed and the manipulator or robot takes back the ladle shroud to allow the removal of the empty ladle and replacement by another ladle filled with a new batch of molten metal. The manipulator or robot (20) repeats the foregoing operations with the new ladle and the same or a new ladle shroud. The manipulator or robot (20) must be operational during the whole duration of the casting of molten metal from the ladle into a tundish, and cannot be used in the meantime for other operations, such as measurements of various process parameters, removal of a clogging in the inner nozzle and the like.
Emergencies may happen, with the gate not functioning properly, requiring the swift removal of the ladle from its casting position to empty the remaining content of molten metal into an appropriate emergency waste area. If the collector nozzle of the ladle (110) is nested in the bore of the ladle shroud (111) with the manipulator or robot firmly gripping the latter in its casting configuration (cf. FIG. 1B), the emergency removal of the ladle would drag therewith both ladle shroud and manipulator or robot, causing serious damages to the installation. Indeed, the manipulator or robot cannot be dragged very far, and the ladle may be blocked halfway, casting molten metal in an inappropriate area of the workshop causing serious consequences and danger.
To prevent such accidents to occur, specific ladle shrouds and coupling mechanisms comprising means for holding them in casting configuration without the need of a manipulator or robot have been proposed in the art. This way, the swift removal of a ladle would certainly break the ladle shroud, but would not drag and be stopped by a bulky (and expensive) manipulator or robot in its run.
For example, JPS09-201657 proposes a ladle shroud provided with coupling means including a bayonet requiring the rotation of the nozzle about its longitudinal axis to block it in its casting configuration. Such rotation can become very difficult as soon as the slightest amount of molten metal flows into and jags the bayonet mechanism upon freezing. Alternatively, JPS09-108825 proposes a ladle shroud comprising two pins on either side thereof suitable for being held in casting configuration by a moving bracket comprising complementary slots for receiving said pins. This mechanism requires an excellent coordination between the loading of a ladle shroud onto the slots of the brackets, and the tilting of the latter in a clamping configuration.
Once a ladle loaded with a fresh batch of molten metal is brought into casting position, it is not always straightforward to initiate the discharge of molten metal into a tundish by opening the gate (114u, 114d). Indeed, when molten metal contacts the relatively cold walls of the vessel it may freeze forming a solid layer against the walls. The freezing of molten metal should be avoided by all means at the levels of the nozzle system and gate, lest the casting operation should be interrupted to unclog the system. Static molten metal has plenty of time to freeze in place at the gate during the transfer of the ladle. For this reason, a plugging material (300), usually sand, is often used to fill the bore of the inner nozzle from its inlet to the closed gate to prevent any molten metal from flowing therein, such that metal freezing and clogging of the nozzle and gate system are prevented. Upon opening the gate, the plugging material flows out followed by the molten metal thus preventing any metal from dwelling and freezing in the inner nozzle (113).
A solid crust of sintered sand impregnated with frozen metal usually forms at the interface between molten metal and sand. In most cases, the crust is thin enough to break under the own weight of molten metal upon opening the gate. Sometimes, however, it may happen that the crust is hard enough to resist the weight of the molten metal. The crust must then be broken or fused with a tool or torch handled manually or with a robot. Because of the length of a ladle shroud, this operation is very cumbersome if the ladle shroud is already coupled to the collector nozzle of the ladle. If the crust resists, a ladle shroud in a traditional installation such as illustrated in FIG. 1B must be de-coupled from the collector nozzle, the crust broken or molten with a torch to initiate the casting of molten metal. Coupling the ladle shroud again to the collector nozzle as metal is flowing through the collector nozzle is dangerous as spilling of molten metal is unavoidable.
To eliminate the need of such dangerous operation, a device for inserting and removing ladle nozzles was proposed in WO2004/052576. Though solving a number of the problems discussed above, said device is, however, cumbersome to operate. The device is rather large in size and does not provide the necessary visibility to permit an operator to work with the high precision required for the installation of a ladle shroud. For example, the lack of clearance with the tilting bar and ribs of the ladle and also between the bottom of the tube and the tundish is a drawback of said coupling assembly.
The present invention proposes a solution solving all the issues raised above, such as providing a ladle shroud that can be inserted and removed easily, which holds in place without the need of any exterior manipulator or robot, and which allows the coupling to a ladle of a short collector nozzle upon initiation of the casting followed by the replacement thereof without spilling of molten metal by a long ladle shroud once casting has successfully initiated. These and other advantages of the present invention are presented in the following sections.